Bio-syncretic robots composed of biological and electromechanical systems

Potential approaches to developing bio-syncretic robots are proposed in different dimensions, including monomial functions and closed loop systems of living cell-based sensing, intelligence, and actuation.

among the biological sensing functions, visual sensing may occupy up to 80% of the external information received by humans; therefore, bio-syncretic visual sensing has been widely researched. Some photosensitive biological materials, such as bacteria and cells, have been used to realize the effective transformation of environmental information into electrical signals based on the electrophysiological response under external stimulation. Moreover, by integrating biological materials with artificial electronic systems, various bio-syncretic sensing systems have demonstrated effective environment sensing [5,6]. Regarding intelligence, distinct neural networks derived from human pluripotent stem cells or isolated from the brain tissue of rats have been cultured in vitro and were combined with silicon computing systems with microelectrode array (MEA) chips to form a bio-syncretic intelligence controller. Living neural networks can be dynamically trained in control processes to realize effective learning and control of simulated and physical robots [7,8]. For actuation, various contractile living materials, such as cardiomyocytes, skeleton muscle cells, and dorsal vascular tissues of insects, have been used as actuators and subsequently integrated with artificial structures to construct bio-syncretic robots with different locomotion modalities, including swimming, walking, and manipulation, etc. In addition, robot control methods based on different mechanisms, such as electrical or optical stimulation, have been explored to realize the controllability of bio-syncretic robots [9,10].
Existing research on bio-syncretic robots has preliminarily verified that it is feasible to use biological materials as robotic function cores. However, the majority have only demonstrated simple functions, meaning that there is much more work to be done in the field to give full play to the functions of biological systems. Thus, further exploration of bio-syncretic robots in terms of sensing, intelligence, and actuation is necessary for their practical application. The potential roadmap for the development of bio-syncretic robots is outlined in Fig. 1.

SENSING: FROM A SINGLE SPECTRUM AND SINGLE PIXEL TO MULTIPLE SPECTRA AND AN ARRAY OF PIXELS
Current bio-syncretic sensors mainly focus on specific single spectrum imaging with a single photosensitive pixel, which greatly restricts sensing applications due to low sensing efficiency and sparse spectral information. To improve imaging performance, first, biological materials able to respond to multiple spectra, including visible and infrared light, should be exploited by engineering powerful variants or advancing co-transfection C The Author(s) 2022. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Natl Sci Rev, 2023, Vol. 10, nwac274  techniques. Second, MEA could be used to realize a sensing array that can detect the optoelectronic signals of ordered photosensitive cells cultured on it. Indeed, nanomaterials such as graphene and molybdic sulfide can be adopted to effectively enhance cellular signals. Finally, image reconstruction algorithms for multichannel cellular signals should be explored based on the imaging mechanism of humans to realize high-efficiency and multispectral imaging.

INTELLIGENCE: FROM A 2D NEURAL NETWORK AND SIMPLE STIMULATION TO 3D NEURAL TISSUE AND ACCURATE TRAINING
Currently, most bio-syncretic intelligence is based on a 2D neural network, which restricts connection complexity and information processing capacity. Furthermore, the MEA used in these works only provides simple records and stimulation for the network, which may be disadvantageous for complex information transmission and accurate network training. On the one hand, increasing the intelligence level of the bio-syncretic controller may depend on an in vitro 3D neural network comprised of multiple layers with the capability of complex information processing. On the other hand, a matching 3D MEA should be developed to realize accurate stimulation and high-throughput information interaction with external devices. The stimulation method, comprising multiple sources, such as electrical, optical, and chemical sources, should also be researched to execute selective training of different cell types and distinct regions. It is worth noting that training with motion feedback may be a potential method to effectively generate intelligence in neural networks. The plastic connections between nerve cells will be intensively regulated by selective stimulation based on the comprehensive information of the motion targets and behaviors of the robot to realize intelligent learning and control.

ACTUATION: FROM MINIATURE CELL CLUSTERS AND SIMPLE MOVEMENT TO MACROSCOPIC TISSUE AND COMPLEX BEHAVIOR
Existing bio-syncretic robots mainly focus on simple motions based on in vitro cultured miniature cell clusters. Although the small size of the current robots is advantageous for special applications, such as medicine and detection in narrow spaces, miniature biological actuators are rarely used in macroscopic robots, thereby restricting the development of bio-syncretic robots on different scales. As such, bioprinting technology is urgently needed to fabricate macroscopic muscle actuation tissues with blood vessels to create large-sized bio-syncretic robots. Indeed, bionic integration inspired by bone and muscle tissues may be a potential approach to the effective rigid-flexible coupling of nonliving structures with the biological actuators of bio-syncretic robots. Moreover, organisms are able to perform complex and flexible behaviors depending on multiple cooperative muscle tissues. Therefore, a robot consisting of multiple biological actuators under synergetic control through artificial or biological interfaces, such as nonliving MEA, microprinted flexible electrodes, and living neuromuscular junctions, should be designed to go beyond simple movements to realize more complex behaviors.
The above in-depth study will effectively improve the performance and application of bio-syncretic function, including sensing, intelligence, and actuation. Current bio-syncretic robots with monomial functions struggle to interact with humans and the environment, while humans are able to process efficient human-human and humanenvironment interaction through the organic combination of sensing, intelligence, and actuation. Therefore, the closed loop of sensing-intelligenceactuation is necessary to drive the development of bio-syncretic robots from a single function to intelligent cooperative behavior (Fig. 1). In the proposed closed-loop bio-syncretic system, each function element of sensing, intelligence, and actuation can be integrated with artificial electromechanical systems by the interfaces of MEA, flexible micro wires, micro printed electrodes, optical fibers, and so on. Moreover, functional elements can also be connected by biological materials, such as nerve fibers and neuromuscular junctions. In this approach, the connection between each function module is based on biological information flow, and the nerve cells of the intelligence unit will be physiologically connected to both the sensing and actuation units. This will not only realize information transfer but also reciprocally enhance the functional performance and lifespan of the bio-syncretic units of sensing, intelligence, and actuation. Therefore, the integration of sensing, intelligence, and actuation will effectively improve the functional systematization and overall performance of bio-syncretic robots.
The proposed development of biosyncretic robots will improve their sensing, intelligence, actuation, and interaction capabilities. Furthermore, bio-syncretic robots may be useful in other fields. For example, in vivo robots actuated by living cells can realize self-propelling motion with bioenergy. Moreover, bio-syncretic robots comprised of human-derived biological materials and electromechanical systems may possess preferable biocompatibility by avoiding the immune response [11].
In addition, the constructed in vitro bio-syncretic systems may be beneficial in better understanding living systems and in medical research and development. However, the further development of bio-syncretic robots relies on progress across multiple academic disciplines, including neuromuscular control mechanisms, robotic autonomous and wireless control methods, integrated biology-mechatronics manufacturing, and biological activity in vitro maintaining technologies, such as vascularized nutrition transport, microfluid chip-based medium renewal, and packaging skin-based living material protection. It is hoped that, in the future, bio-syncretic robots may realize a heightened coexisting-cooperative-cognitive working mode with humans and the environment, allowing them to serve society more safely and efficiently.